Don't rely on patch being available on the build host. Run the patch

command in the container.

fastpfor/Makefile

@@ -3,19 +3,18 @@ SHELL:=/bin/bash
 ubuntu1804:
 	git clone https://github.com/lemire/FastPFor
 	pushd FastPFor && git checkout 8030e3808f857cc12d1d09e264f1ad2a05b1964a # master at the time of writing
-	patch FastPFor/CMakeLists.txt libcpp.patch
 	bash ../ubuntu1804.sh make impl
 	make package-ubuntu1804
 
 ubuntu2004:
 	git clone https://github.com/lemire/FastPFor
 	pushd FastPFor && git checkout 8030e3808f857cc12d1d09e264f1ad2a05b1964a # master at the time of writing
-	patch FastPFor/CMakeLists.txt libcpp.patch
 	bash ../ubuntu2004.sh make impl
 	make package-ubuntu2004
 	bash ../ubuntu2004.sh chown $$(id -u):$$(id -g) . -R  # fix permissions
 
 impl:
+	patch FastPFor/CMakeLists.txt libcpp.patch
 	update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang++-10 40
 	update-alternatives --install /usr/bin/cc cc /usr/bin/clang-10 40
 	update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-10 40